Liquid developer for electrophotography containing stain texture preventing agent

ABSTRACT

THE INVENTION RELATES TO A LIQUID DEVELOPER FOR ELECTROPHOTOGRAPHY, COMPRISING A CARRIER LIQUID AND A TONER DISPERSED IN SAID CARRIER LIQUID, WHEREIN SAID DEVELOPER CONTAINS AT LEAST ONE COMPOUND SELECTED FROM THE GROUP CONSISTING OF METAL-DIALKYL-DITHIOPHOSPHATES, SODIUMALKYL-PHOSPHATES, ALKY-PHOSPHATES, ALKALI METAL-ALKYLSULFATES, ALCOHOLS, MONOCARBOXYLIC ACIDS, PHTHALIC ACID, ALKYL PHTHALATES, AMMONIA, AMINES, ALDEHYDES, DEKALIN, AZOBENZENE, CAMPHOR, CITRAL AND SYRENE.

United States Patent LIQUID DEVELOPER FOR ELECTROPHOTOG- RAPHY CONTAINING STAIN TEXTURE PRE- VENTIN G AGENT Zenjiro Okuno and Akinori Mizuno, Tokyo, Japan, assignors to Kabushiki Kaisha Ricoh, Tokyo, Japan No Drawing. Filed May 22, 1969, Ser. No. 827,049 Claims priority, application Japan, May 30, 1968, 43/ 36,976; Mar. 25, 1969, 44/22,091 Int. Cl. G03g 9/04 US. Cl. 252--62.1 3 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION (1) Field of the invention The present invention relates to a liquid developer for use in the wet-developing process of electrophotography.

(2) Description of the prior art According to the prior art, the copying material for use in electrophotography is generally manufactured by forming a photoconductive layer on the surface of a support sheet or base, such as a metallic plate or paper whose back has been processed for conductivity, by means of coating said surface with a dispersion prepared through dispersing fine particles of some photoconductive substance, for instance, zinc oxide powder, in an organic solution of resinous binder, and drying the thus coated surface. In copying material manufactured in this manner, it is common that the photoconductive substance contained in the photoconductive layer fails to be uniformly dispersed therein, and also that the surface of said layer is considerably uneven.

When the copying is effected by employing a copying material as above and carried out according to conventional electrophotography, especially in case of copying a wet-type electrophotograph to be developed with a liquid developer, there results the stained texture owing to the lack of smoothness of the surface of photoconductive layer as well as lack of uniformity of the layer itself. And, the occurrence of said stained texture has been all the more conspicuous in case of the negativeto-positive type developing method which comprises the steps of imparting an electrostatic latent image by employing a negative original onto 2. copying material charged with electricity and subsequently developing with a developer containing a toner charged with electricity in the same polarity as the electric charge of said electrostatic latent image area.

The cause of said stained texture has been analyzed as follows. That is, inasmuch as the copying of an electrophotograph is generally effected by forming the image using a toner contained in the developer, said toner adsorbing electrostatically onto the copying material in proportion to the density of electric charge impressed on the photoconductive layer of said copying material, to wit, the highness of potential (or strictly speaking, the intensity of electric field), a photoconductive layer is required to be so charged as to have a uniform potential 3,681,243 Patented Aug. 1, 1972 when imparted an electrostatic photosensitivity by means of impressing an electrostatic charge thereon. However, those conventional elecrtophotographic copying materials on the market can hardly be expected to be uniformly charged because of their lacking of smoothness of surface and lack of uniformity of the layer as described in the foregoing. This lack of uniformity in charged potential causes the toner to adsorb even in the non-image areas to thereby bring about the stained texture. Accordingly, when a negative original is laid on a photoconductive layer for exposure, the potential of the image area of said original is rapidly extinguished to form a comparatively large area of low potential, but there is unevenness in the residual potential of the non-image area. Consequently, if a copying material having such an uneven potential is dipped in a negative-to-positive type developer, the toner in said developer will adhere not only preferentially onto the image area of the negative whose potential is extinct but also onto the low-potential portions of the non-image area thereof, to thereby bring about said strained texture.

SUMMARY OF THE INVENTION The object of the present invention is to provide a liquid developer for use in electrophotography, said developer precluding the possibility of bringing about a stained texture in case of developing an electrophotographic copying material which has essentially such a defect as set forth above.

A liquid developer for electrophotography according to the present invention contains a carrier liquid wherein a toner consisting of pigment and polarity-controlling resin has been dispersed, and further contains at least one member selected from the group consisting of such substances as expressed by the following general Formulas a through j, Dekalin, azobenzene, camphor, citral and styrene monomer.

General Formula a P Ruo \SMG1S/ \ORXV (metal-dialkyl-dithiophosphate) wherein R R R and R each represents a monovalent radical selected from the group consisting of alkyl radicals having 3 to 18 carbon atoms and aralkyl radicals combined with a side-chain having 3 to 18 carbon atoms, and Me represents a divalent metal radical.

General Formula b o (RO)PEONa ONa (disodium-alkyl-phosphate) wherein R represents a radical selected from the group consisting of H and alkyl radicals (C H having 2 to 12 carbon atoms.

General Formula c 0 (R-O)2P ONa (sodium-dialkyl-phosphate) wherein R represents the same radical as R in the foregoing.

General Formula (1 R' O) P=O (trialkyl-phosphate) wherein R represents the same radical as R in the fore going.

General Formula e (alkalimetal-alkyl-sulfate) wherein R represents a member selected from the group consisting of alkyl radicals (C H having 12 to 16 carbon atoms, and Me represents a member selected from the group consisting of Na and K.

General Formula f c,,H,,,,,oH (alcohol) wherein n is the numerical value of 4n 16.

General Formula g C H COH (monocarboxylic acid) wherein n is the numerical value of 11511515 General Formula h OOOX -COOX (phthalic acid and alkyl phthalate) wherein X represents a radical selected from the group consisting of H and alkyl radicals (C H l having 1 to 4 carbon atoms.

General Formula i (ammonia and amines) wherein R R and R each represents one radical selected from the group consisting of H and alkyl radicals (C H t having 2 to 17 carbon atoms.

General Formula j (aldehyde) wherein A represents a radical selected from the group consisting of [of which X and Y each expresses one radical selected from the group consisting of -H, -OH, NO -N(CHa)z, -OCH and -CHCH and C H These substances, when dissolved and retained in a liquid developer for electrophotography, have the effect of providing said developer with a superb efliciency preventing the occurrence of stained texture on the occasion of developing the copying material.

The exact mechanism involved in preventing or controlling the occurrence of stained texture at the time of development as displayed by said substances represented by metal-dialkyl-dithiophosphates according to the present invention, is not known. However, according to the observation by the inventors of the present invention, said action in case of the negative-to-positive type development can be explained as follows.

By way of example, an electrophotographic copying material having an electrostatic latent image formed thereon is dipped in a liquid developer wherein metal-dialkyldithiophosphate has been dissolved. Thereupon, molecules of said metal-dialkyl-dithiophosphate are influenced by the electric field and cause polarization as follows:

Rno S-IAKP-S \ORUI And, in case the residual electric charge of, for example, the non-image area of a copying material is negative, said molecules are repelled electrostatically and migrate to the relatively low potential portion or the non-potential portion to be adsorbed thereon, and the absorption continues until said portions come to have a potential equivalent to that of their surroundings. Further, as it is conceivable that the molecules of salt are greatly polarized at a portion having high density of electric charge of the copying material, they may be preferentially adsorbed onto the minute portions having low potential in the vicinity of said portion with high density of electric charge. Accordingly, inasmuch as the relatively large image-area whereon the toner is supposed to be adsorbed has an electric charge of quite low density, polarization of said metal-dialkyl-dithiophosphate at this portion can hardly take place, and as a result the adsorption of molecules thereof comes to be nil or quite slight, so that there is no fear of encountering an obstacle in forming images by virtue of said toner. Further, as it is imaginable that the same phenomenon as above may present itself in case of those substances other than said metaI-dialkyl-dithiophosphate, too, they are considered as contributable to preventing the stained texture from taking place.

As for the divalent metals to be combined into said metal-dialkyl-dithiophosphate expressed by the foregoing general Formula a and employed for a developer in the present invention, zinc, barium, calcium, strontium, magnesium and the like are suitable. And, as for the preparation of the metal-dialkyl-dithiophosphate as expressed by said general Formula a, there have heretofore been proposed a variety of modes thereof, and said salt is capable of being easily synthesized on the basis of descriptions pertaining to the examples of embodiment appearing in the specifications of, for instance German Patent No. 1,018,428 US. Patent No. 2,838,555 US. Patent No. 2,850,452 US. Patent No. 2,895,973 British Patent No. 822,655 US. Patent No. 2,934,614 British Patent No. 2,850,453

and, therefore, any further elucidation in this respect is unecessary.

In addition to the above enumerated salts, there are a variety of compounds with various trade names as follows which comprise some of these salts as principal component, respectively, and have been used commercially as additives for engine oil, and any of them is also suitable for the present invention.

Trade name Name of manufacturer Santolube 393 Monsanto Chemical Co. Amoco 193 Amoco Chemical Co. OLOA 267 Olonite Chemical Co.

Concrete examples of those substances to be expressed by the aforesaid general Formulas b through j are as enumerated in examples appearing hereunder, and any of the following substances is suitable for the present invention: to wit, any one of phosphates such as disodiummonolauryl-phosphate (general Formula b), sodiumdilauryl-phosphate (general Formula 0) and triethylphosphate (general Formula (1), sulfate such as sodiumlauryl-sulfate (general Formula e), alcohols (general Formula f) such as nonyl alcohol, isopropyl alcohol, ,3, Naphthal and cetyl alcohol, monocarboxylic acids (general Formula g) such as lauric acid, myristic acid and palmitic acid, phthalate (general Formula h) such as butylphthalate, amines (general Formula i) such as triethylamine and stearyl amine, aldehyde (general Formula i) such as n-butylaldehyde, cinnamic aldehyde, salicylaldehyde, dihydroxybenzaldehyde, di-nitrobenzaldehyde, dimethylaminobenzaldehyde, dimethoxybenzaldehyde and tolualdehyde.

And, the developer of the present invention is obtained by means of kneading at least one of the foregoing substances with pigment, one or more than two resins contributable to imparting polar-controllability, dispersibility and fixability, and a small quantity of organic solvent, and subsequently dispersing the thus kneaded mixture in a carrier liquid (viz. paraflinic hydrocarbon or isoparaflinic hydrocarbon having a electric resistance of more than Q-cm. and a dielectric constant of less than 3). The alternative to said means is to add at least one of the aforesaid substances to either a concentrated liquid developer (prior to dispersion in a carrier liquid) put on the market or a liquid developer for immediate use as available also in the market and mix them well. The suitable quantity of said substance to be employed or the content in the developer in this occasion is in the range of 0.001- 0.1 part by weight per 1 part by weight of pigment of a toner contained in the developer. In this connection, it is noted that the quantity of phosphates as expressed by general Formulas b, c and d and the quantity of sulfate as expressed by general Formula e for adding to the developer are sufiicient in said range for the sake of effecting a satisfactory result, but as for other substances than the foregoing, the optimum quantity to be added is in the range of 0.01-0.1 part by weight for dithiophosphates as expressed by general Formula a, 0.01-0.05 part by weight for phthalate, 0.001-0.005 part by weight for alcohols, 0.01-0.l part by weight for carboxylic acid, 0.001-0.005 part by weight for amine, .00l-0.01 part by weight for Dekalin, 0.01-0.05 part by weight for azobenzene, 0.05- 0.1 part by weight for camphor, 0.0050.05 part by weight for citral, 0.001-0.05 for styrene monomer and 0.001- 001 part by weight for aldehydes, respectively.

When an electrophotographic copying material is developed with the aforesaid liquid developer according to the present invention, the so-called stained texture incident to development can be almost completely eliminated and, as a result, formation of a fully clear image can be expected. Further, the developer of the present invention has such an advantage that, despite the concentration of image brought about thereby is quite evenly matched with that developed with conventional developers, the resultant image is visually more highly concentrated than the one in the prior art because of the absence of stained texture.

The following is a further elucidation of the present invention with reference to embodiments thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 7 Example 1 A mixture having a composition consisting of was thoroughly mixed for 16 hours by means of a ball mill, and subsequently 5 g. of thus treated mixture was dispersed in 1,000 cc. of a carrier liquid (a liquid hydrocarbon manufactured by ESSO Standard Sekiyu Co. and having the trade name Naphtha No. 6), whereby was obtained a liquid developer for use in electrophotography, said developer containing pigment particles negatively charged. Further, for the sake of comparison of efficiency, another liquid developer for use in said comparison was prepared by similarly dispersing 5 g. of a mixture having the same composition as above but with no metal-dialkyldithiophosphate.

Meanwhile, an electrostatic latent image was imparted onto commercially available electrophotographic copying materials through the steps of impressing thereon a negative electric charge according to conventional electrophotography and then laying the negative image upon thus impressed copying materials for subsequent exposure by virtue of a tungsten-filament lamp. After that, the copying materials thus charged with an electrostatic image respectively were employed for forming a copied image by dipping in the liquid developer according to the present invention on the one hand, and bydipping in the developer prepared for the comparisons sake on the other hand, for 25 seconds each. The measurement of the resultant concentration of image area and that of non-image area vifii tlie area of stained texture) has led to the following TABLE 1 Concentration 01- Non-image Developer Image area area Developer under the present invention 1. 36 0. 13 Developer prepared for comparisons sake.. 1. 20 0. 23

In this connection, it is noted that each value of the concentration in the foregoing Table 1 was measured through the relative measurement method, wherein the printing was conducted by employing a 2 mm.-square discontinuous gray scale as the original to avoid the edge effect peculiar to electrophotography and the maximum concentration of the print was measured by comparing it with one having a known concentration.

Example 2 A mixture having a composition consisting of- Carbon black Polar-controlling resin: Nikanol HP- 30 Resin contributing to dispersibility: Plexol #966 (an acrylic copolymer resin manufactured by Rohm & Haas Co.)

Toluene 5%-toluene solution of metal-dialkyl-dithiophosphate (as expressed by general formula wherein both R and R are isopropyl radicals, both R and R are isobutyl radicals, and Me is barium) 10 Example 3 A mixture having a composition consisting of 5%-toluene solution of metal-dialkyl-dithiophosphate (as expressed by general formula wherein both R; and R are isobutyl radicals, both R and R are lauryl radicals, and Me is calcium) 10 was treated in the same way as Example 1, whereby was obtained a liquid developer for electrophotography.

developers. On the other hand, for the purpose of comparing the efliciency of developers, a developer for use in comparison was prepared by means of dispersing about 5 g. of the mixture having the aforesaid composition in a carrier liquid. Each of the above developers was then treated in the same way as Example 1 and employed for developing the electrophotographic copying material carrying the electrostatic latent image, and subsequently measurement was conducted of the concentration of the resultant images as well as the concentration in the texture area.

The above-stated compositions of developers and the results of measurement with respect to the efficiency of those developers are as put together, in the following Table 2. In this context, it is noted that, notwithstanding there was employed-for respective developer according TABLE 2 Quantity added to Concentration 01- basic Developer onlposi- Image Texture number Name of Additive tion (g-) area area 1 Triethyl phosphate (5% toluene solutio 5 1. 29 0.20 2 Monolauryl phosphate (5% toluene solution; a manufacture of 5 1.33 0,11

Nikko Chemicals, Inc., with the trade name Nikkol-SLP). 3 Dilauryl phosphate (1% toluene solution) 1 1. 24 0.09 4. Dilauryl phosphate (5% toluene solution; a manufacture of 5 1.32 0. 13

Nikko Chemicals, Inc., with the trade name N ikkol-DLP- l). 5 Dilauryl phosphate (5% toluene solution) 1 1. 29 0. 09 6-- --do 10 1. 29 0. 11 7 Na-lauryl phosphate (5% toluene solution; a mannia 5 1. 31 0, 11

Nikko Chemicals Inc., with the trade name N ikkol-SLS). 8 K-lauryl sulfate (5 toluene solution; a manufacture of Nikko 1.32 0. 11

Chemicals, Inc., with the trade name Nikkol-KLS). 9 N a-cetyl sulfate (5% toluene solution; a manufacture of Nikko 5 1.27 0.09

Chemicals, Inc., with the trade name Nikkol-SCS). 10 Na-lauryl sulfate (1% toluene solution) 1 1. 30 0. 11 11.-- Na-lauryl sulfate (5% toluene solution) 1 1. 30 0. 11 12 -do 10 1.28 0.11 13 Monomcthyl phthalate (5% toluene solution).-. 1 1. 34 0.15 14.-- Dibutyl phthalate (5% toluene solution) 5 1.34 0. 11 15... -do 1 1.34 0.11 16--. Phthalic acid (5% toluene solution) 1 1. 27 0. 13 17--. Isopropyl alcohol (0.1% toluene solution) 5 1.28 0. 09 18 "do 1 1.26 0.09 19... Getyl alcohol (0.1% toluene solution) 5 1. 30 0. 11 20--- Nonyl alcohol (0.1% toluene solution) 5 1. 0. 13 21 Lauricacid (5% toluene solution) 5 1. 40 0. 11 22 Myn'stic ac d (5% toluene solution) 5 1. 38 0.09 23 Palmitic acid (5% toluene solution) 5 1.30 0.11 24.-. .d0 1 1. 31 0. 13 25--- --do 10 1. 0.11 26--- Triethyl amine (0.1% toluene solution) 5 1.24 0. ll 27-.- Stearyl amine (0.6% toluene solution).- 1 1.25 0. 11 28-.. do 5 1.26 0.11 29 Dekalin (5% carbon tetrachloride solution) 5 1. 33 0. 11 30-.- Dekalin (1% carbon tetrachloride solution) 5 1. 29 0. 11 31 Dekalin (0.1% carbon tetrachloride solution)--. 5 1. 28 0.09 32.-- Azobenzene (5% toluene solution) 5 1.26 0. 09 33 Azobenzene (1% toluene solution) 5 1. 27 0. 11 34. Camphor (5% carbon tetrachloride solution)- 5 1. 30 0. 11 35 10 1.30 0.09 36 Citral (5% toluene solution) 5 1 2g 11 37 1 1.28 0.09 38 Citral (1% toluene solution) 1 1. 28 0. 11 39 Styrene monomer (5% toluene solution).. 1 1. 33 0. 09 40 .d0 5 1.30 0.13 110..-.-.. 0.1 1. 25 0.11 42 Butyl aldehyde (0.1% toluene solution) 0. 1 1. 28 0. 09 23) Cinnamic aldehyde (6% toluene solution) 0.1 1. 23 0.09 1 1.20 0.23

1 Developer for comparison.

Thus prepared developer displayed a developing effect equivalent to those developers according to the present 0 invention as employed in the foregoing examples.

Example 4 was combined with various additives respectively, and subsequently about 5 g. each of the respective mixtures thus prepared was dispersed in the same carrier liquid as Example 1, whereby were prepared 43 kinds of liquid to said Table 2just one additive substance effective in preventing the stained texture, it is definite that, in so far as the quantity employed is in the range of 0.00 1-0.1 part by weight per 1 part by weight of the pigment contained in a developer, the effect to be brought about in case two substances or more are employed is equivalent to that in case just one substance is employed.

What we claim is:

1. A liquid developer for electrophotography comprising l) a carrier liquid consisting of a liquid hydrocarbon having an electric resistance of more than 10 Q cm. and a dielectric constant of less than 3; (2) a toner dispersed in said carrier liquid, said toner being comprised of carbon black particles and a coating layer coated on said particles, said coating layer consisting essentially of a resin selected from the group consisting of phenol modified xylene resin and acrylic resins; and (3) a stain texturepreventing agent dissolved in said carrier liquid, said agent being selected from the group consisting of compounds expressed by Formula a R; O S S Rm in a quantity in the range of 0.01 to 0.1 part by weight per one part by weight of said carbon black, and wherein each of R R R and R is a monovalent radical selected from the group consisting of alkyl radicals having 3 to 18 carbon atoms and aralkyl radicals combined with a side-chain having 3 to 18 carbon atoms, and Me; is a divalent metal radical; compounds expressed by Formula b R (Bl-O) l ONa in a quantity in the range of 0.001 to 0.1 part by weight per one part by weight of said carbon black, and wherein R is selected from the group consisting of H and alkyl radicals having 2 to 12 carbon atoms; compounds expressed by Formula 0 in a quantity in the range of 0.001 to 01 part by weight per one part by weight of said carbon black, and wherein R is the same as R in the preceding Formula b; compounds expressed by Formula (1 in a quantity in the range of 0.001 to 0.1 part by weight per one part by weight of said carbon black, and wherein R is an alkyl radical having 12 to 16 carbon atoms, and Me is a member selected from the group consisting of Na and K.

2. A liquid developer according to claim 1, wherein said agent is a compound of the Formula a and its content in said carrier liquid is in the range of 0.01-0.1 part by weight per part by weight of said carbon black.

3. A liquid developer according to claim 2, wherein the metal of the compound expressed in the general Formula a is a member selected from the group consisting of zinc, barium, calcium, strontium 'and magnesium.

References Cited UNITED STATES PATENTS 3,241,957 3/1966 Fauser et al. 25262.1 3,079,272 1/1963 Greig 252-62.1 3,512,965 5/ 1970 Matkan 25262.'1 3,444,083 5/ 1969' Oliphant 252-621 3,076,722 2/1963 Greig 25262.1 3,078,231 2/1963 Metcalfe et a1. 252-62.1 3,507,679 4/1970 Metcalfe et a1. 252--62.1 3,528,097 9/1970 Otsuka et a1 25262.1

GEORGE F. LESMES, Primary Examiner J. P. BRAMMER, Assistant Examiner US. Cl. X.R. 117--37 LY 

